What makes a graph more accurate?

A graphical form that involves elementary perceptual tasks that lead to more accurate judgments than another graphical form (with the same quantitiative information) will result in better organization and increase the chances of a correct perception of patterns and behavior.

Bar chart

Pie chart

Statistical maps with color

Curvature

Curvature

Experimental design

• \(N = 55\)
• Experiment 1 - asked to make assessments of length and position along a common scale
• Experiment 2 - asked to make judgments of position and angle (pie vs bar chart)
• Make visual assessments of what percentage one value was of a larger value

What we learn

• Use theory and experiments to develop rules for data visualization
• Different perceptual tasks are easier or harder to decode
• Pick the highest-level encoding task possible given the data structure/purpose of the visualization

Amazon Mechanical Turk

In defense of pie charts

• Which is easier to distinguish: line length or area, angle, and arc length?

• Psychophysical theory of perception:

  \[\text{Subjective area} = \text{Area}^.86\]

• Focus more on the subarea

• What is the point of the chart?

Takeaways

• For pure magnitude identification, bar charts are superior
• For comparing percentages, either chart is acceptable
• To compare combinations of groupings, pie charts are slightly superior
• Tables are only useful if you want to report exact percentages